This invention relates to an improved process for preventing scale formation and enhancing corrosion inhibition in aqueous systems by the addition of small quantities of low molecular weight, water-soluble polymers. More particularly the invention relates to the selection of monomers which when polymerized together form polymer compositions that exhibit unexpectedly improved precipitation inhibition and corrosion inhibition for aqueous systems, such as cooling waters, that contain phosphates, iron, zinc and other scale-forming salts.
Many industrial applications and residential areas utilize water containing relatively high concentrations of inorganic salts. These salts are formed by the reaction of metal cations, such as calcium, magnesium or barium, with inorganic anions such as phosphate, carbonate and sulfate. These salts have low solubilities in water and as their concentration in solution increases, or as the pH or temperature of the water containing them increases, these salts tend to precipitate from solution, crystallize and form hard deposits or scale on surfaces. Scale formation is a problem in heat transfer devices, boilers, oil recovery operations and on clothes washed with such hard waters.
Many cooling water systems, including industrial cooling towers and heat exchangers made from carbon steel, experience corrosion problems caused by the presence of dissolved oxygen. Corrosion is combated by the addition of various inhibitors such as orthophosphate, polyphosphate and zinc compounds, alone or in combination. The addition of phosphates, however, adds to the formation of highly insoluble phosphate salts such as calcium phosphate. The addition of zinc compounds can also lead to the precipitation of insoluble salts such as zinc hydroxide, and zinc phosphate. Other inorganic particulates, such as mud, silt and clay, are commonly found in cooling water. These particulates tend to settle onto surfaces and thereby restrict water flow and heat transfer unless they are effectively dispersed.
Processes that would benefit from an additive that would inhibit the precipitation of scale-forming materials include, for example, cooling water, boiler water, geothermal process water, sugar processing and desalting operations using distillation processes. In each of these processes, heat is transferred to or from the water and can aggravate precipitation problems in a variety of ways.
The stabilization of aqueous systems containing scale-forming salts involves one or more mechanisms. Anti-precipitation involves the delay of precipitation by limiting the size of the salts to sub-colloidal dimensions possibly by adsorption of the inhibitor onto the salt crystal soon after nucleation, thereby interfering with further crystal growth. Another stabilization mechanism involves the ability of the inhibitor to interfere with and distort the crystal structure of any scale formed making the scale more easily fracturable and dispersable.
A variety of methods have been employed to stabilize aqueous systems. Polymers derived from (meth)acrylic acids and salts as well as mixtures of such polymers with other compounds and polymers have been used as precipitation inhibitors for aqueous systems.
U.S. Pat. No. 3,709,815 discloses polymers having molecular weights greater than 20,000 containing 15-100 mole percent 2-acrylamido-2-methyl-1-propanesulfonic acid/0-85 mole percent acrylic acid or acrylamide for dispersing calcium phosphate precipitates at room temperature and proposes the polymers for use in high temperature boiler systems.
U.S. Pat. No. 3,806,367 discloses polymers having molecular weights from 1,000 to 100,000 containing 20-80% 2-acrylamido-2-methyl-1-propanesulfonic acid for dispersing precipitates at 25.degree. C. in aqueous systems.
U.S. Pat. No. 3,928,196 discloses polymers having molecular weights from 1,000 to 10,000 containing 5-75 mole percent 2-acrylamido-2-methyl-1-propanesulfonic acid/25-95 mole percent acrylic acid for inhibiting calcium sulfate and calcium carbonate precipitation at 65.degree.-66.degree. C.
U.S. Pat. No. 4,640,793 discloses the use of a mixture of 20-80% (meth)acrylic acid/20-80% 2-acrylamido-2-methyl-1-propanesulfonic acid polymers having molecular weights up to 25,000 together with (meth)acrylic acid polymers for inhibiting calcium sulfate, calcium carbonate or calcium phosphate precipitation at 60.degree.-65.degree. C. in aqueous systems.
U.S. Pat. No. 4,711,725 discloses terpolymers having molecular weights from 3,000 to 25,000 containing 10-84% (meth)acrylic acid/11-40% 2-acrylamido-2-methyl-1-propanesulfonic acid and 5-50% third monomer for inhibiting the precipitation of calcium phosphate at 70.degree. C. in aqueous systems.
Despite the large number of publications disclosing scale inhibition, dispersion and/or stabilization of aqueous systems utilizing polymers derived from (meth)acrylic acid, acrylamidoalkylsulfonic acids and other ethylenically unsaturated monomers at temperatures up to 70.degree. C., no single conventional material or combination has been found to be completely effective at low use levels for stabilizing aqueous sytems containing all the various types of scale-forming salts commonly found in cooling tower waters operated under high temperature conditions, for example, greater than about 80.degree. C., especially in the presence of iron and phosphate salts. No one had, heretofore, discovered that if certain known materials, used for these applications, were polymerized together in certain selected weight ratios to form a low molecular weight, water-soluble polymer composition, the best high temperature stabilization performance characteristics of each constituent could be achieved.
The present invention seeks to overcome the problems of the prior art by providing an improved process for stabilizing aqueous systems at high temperature conditions.